1. Field of the Invention
The present invention relates to a method for manufacturing a resin-coated carrier for an electrophotographic developer for use in a copying machine, a printer or the like, and to an electrophotographic developer using therein the carrier obtained by said method.
2. Prior Art
A two-component type developer used for electrophotography is composed of a toner and a carrier. When the developer is used, the carrier is stirred and mixed with the toner in a development box to give a desired charge to the toner and then carries the thus charged toner onto electrostatic latent images on a photosensitive material to develop the latent images so as to form toner images. The carrier thus used remains on a magnet roll and is then returned again to the development box when it is again stirred and mixed with a fresh toner for repeated use in a developer.
Accordingly, the carrier is required to always provide the toner with the desired charge during the use of the carrier.
Further, in substitution for many carriers, there has recently been generally used a resin-coated carrier prepared by coating the surface of a carrier core with various resins to obtain a carrier which is improved in durability and resistance to environmental dependence.
However, conventional developers which have heretofore been used will cause various inconveniences with respect to images developed, due to collision of carrier particles with each other by stirring, or friction between the development box and the carrier particles. One of the inconveniences is that the toner particles are fusion adhered onto the surface of the carrier particles by friction to cause useless consumption of the toner (the useless consumption being hereinafter refered to as "spent"-phenomenon) so that the distribution of charge amount between the toner and carrier is widened with the result that toner scattering and image fog arise during the use of the developer for testing the service life thereof. Further, in the case in which a resin-coated carrier is included instead of the above carrier in the developer, the resin of the outermost layer of the coated resin falls off to expose the internal resin layer so that the charge amount between the toner and carrier is varied with the result that such toner scattering and image fogging as above arise especially under high-temperature and high-humidity conditions, during the use of the developer. When the developer service life test is further conducted, the resin layer is peeled off to expose the carrier core so that the carrier resistance is varied with the result that the developer cannot maintain its performance of making images which are substantially equal to the initially made image and it finally ends its service life.
In the case in which a developer including the resin-coated carrier is used, a "spent"-phenomenon with the toner tends to decrease as compared with the use of the uncoated carrier in the developer. However, the so-called granulation of the resin-coated carrier particles arises due to the carrier core particles being adhered to one another with the resin at the time of resin coating. The granulated resin-coated carrier particles are then separated one by one from one another so that the fluidity of the developer during printing or copying, is varied with the result that the toner concentration is varied, the density of imagees to be obtained decreases due to the toner scattering caused by excessive concentration of the toner and to lessening of the toner density, and image deficiencies are very often incurred by adhesion of the carrier particles to the image.
There have heretofore been proposed various resin-coated carriers in order to improve carriers in durability. For example, there have been proposed carriers prepared by coating carrier cores on the surface with a fluorocarbon-based polymer (for example, Japanese Patent Application Laid-Open Gazette No. Sho. 54-126,040). Although the resin-coated carrier so proposed was stable in charging characteristics initially, it allowed a developer containing it to vary in fluidity due to the use of the carrier for a long time thereby forming images having insufficient density, impairing a photosensitive material due to adhesion of the carrier thereto and consequently incurring defects as to images, thus the resin-coated carrier having been overall estimated to have a short service life.
Further, carriers coated with a silicone resin or the like have been proposed (Japanese Patent Application Laid Open Gazette No. Sho. 57-78,552, etc.). This silicone resin-coated carrier has an advantage that the use makes it difficult to cause a "spent"-phenomenon because of its low-surface energy. However, it causes fogging and toner scattering due to its gradual decrease in charge level with the result that a developer having the silicone resin-coated carrier has a short service life.
Recently, a carrier has been required to have satisfactory durability and a developer including such a carrier is ultimately required to be maintenance-free for use, as mentioned above. However, the service life of the resin-coated carrier is still short and, therefore, there has remained an outstanding problem as to difficulties in prolonging the service life of a developer containing such a carrier. Further, there has also remained a problem as to a still high cost for coating a carrier core with a resin.
Most of the conventional heating methods involve an external heating system so that when the resin-coated carrier is heated by way of combustion flame, heated air, burner, electric heater, or hot steam, it is slowly heated from the outermost resin layer thereof down to the core material through heat conduction.
Therefore, the resin thus molten in the outermost layer is fused to each other thereby forming granules, and it takes a lot of time to sufficiently heat a portion inner than the resin layer or the core material. Further, due to these reasons, a difference in curing between the inner resin and outer resin, or a loosening of adherence between the resin and the core material may be caused to occur, so that the resin is likely to be peeled off, and the service life of the developer is shortened.
Moreover, the conventional external heating system is also defective in that the baking time is rather long, and the manufacturing cost is high.
The present invention was made to solve the problems of the above mentioned prior art, and an object of the present invention is to provide a method for manufacturing resin-coated carriers for an electrophotographic developer which are excellent in durability, resistance to environment without changing their electrifiability for the toner, and producibility, as well as in economy, and also to provide an electrographic developer including the carrier obtained by said method.